Electrophotography is, as described in U.S. Pat. Nos. 2,297,691 and 2,357,809, for example, to reproduce an image to be in the form of a copied image in such a manner that; an electrostatic latent image of the image is formed on the surface of a photoreceptor; the latent image is transformed into a toner image with a dry type developer comprising colored particles; next, the toner image is transferred to a transfer sheet such as a sheet of paper; and the transferred toner image is permanently fixed by applying heat, pressure or the like; so that a copy of the image may be formed. On the other hand, the photoreceptor from which the toner image was transferred may be reused successively for forming the next images, after the toner remaining on the photoreceptor surface is cleaned up by a cleaning member.
As for the photoreceptors applicable to such an electrophotography as described above, there are well-known photoreceptors including, for example, inorganic photoreceptors such as those of a selenium, zinc oxide, cadmium sulfide and so forth, and organic photoreceptors comprising a high or low molecular weight compound such as polyvinyl carbazole and so forth. In the selenium photoreceptors, however, there are some problems such as that a crystallization is apt to occur in the conditions of a high temperature. therefore, the heat resistance is insufficient and the characteristics such as sensitivity are deteriorated, so that images may be degraded to be unsharp. In the zinc oxide or cadmium photoreceptors, the light-sensitive characteristics thereof are apt to be deteriorated faster by exposing images to light and fog is produced, so that an unsharp image may come out and the durability of the photoreceptor is deteriorated and, in addition, the toxicity to human body is blamed thereon.
In contrast to the above, the organic photoreceptors comprising organic semiconductors are desirable photoreceptors, because they have not the above-mentioned defects, but the advantages such as an excellent layer forming property, an inexpensive manufacturing cost, a high sensitivity, a stable durability, an excellent heat resistance, no toxicity to human body and so forth.
As for the electrostatic latent images formed on the surface of the above-mentioned organic photoreceptor, a negatively charged electrostatic latent image is generally used. This is because there are many kinds of photoconductive substances available for making an organic photoreceptor and a high performance may be displayed.
A negatively charged electrostatic latent image formed on the surface of the above-mentioned organic photoreceptor is developed with such a generally known developer as single-and two-component type developers. The former, the single-component type developers, consist of only magnetic toner containing magnetic substances dispersed in the binders thereof, and the latter, the two-component type developers. are comprised of both toners and carriers comprising magnetic particles.
When a development is carried out with such a developer as mentioned above, the toners constituting the developer should be charged to positive polarity that is the reverse polarity of the negatively charged electrostatic latent image formed on the surface of the photoreceptor.
With the purpose of charging the toner to the positive polarity, a positive charge controlling agent may be added, or, in the case of using a two-component type developer, toners may be frictionally charged to positive polarity by making use of selective carriers.
However, simply with a positive charge applied to the toner, excellent images may not stably be provided extending over a long time. To be more concrete, toners are generally charged by rubbing them with other friction-electrifying member. However, when repeating such operation a number of times, the toner components are partially transferred to the friction-electrifying member by the friction, and the toner components stain the friction-electrifying member surface to which a proper frictional charge should be applied. Therefore, it gradually becomes difficult to apply proper frictional charge to the toner and the absolute value of the frictional charge of the toner is lowered thereby, so that fog is apt to be produced and, consequently, the toner durability is deteriorated.
For the purpose of solving the above-described problems. the following attempts have been made.
(1) In Japanese Patent Examined Publication No. 22447-1978 and Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 6623-1978, there are the descriptions of such an attempt that toners are applied with a positive chargeability by containing inorganic fine particles treated with an aminosilane coupling agent. However, from the results of the studies thereof made by the present inventors, it was proved that, such a developers may be able to display a somewhat good performance only in the initial stage though, the characteristics of the developer are deteriorated as they are repeatedly used a number of times, namely, the chargeability of the toners are apt to be lowered to produce fog or to fly the toners, so that images are stained. Further, in the conditions of a high temperature, the above-mentioned defects become more serious.
(2) In Japanese Patent O.P.I. Publication Nos. 123550-1981 and 34539-1984, there are the descriptions of such an attempt that toners have been tried to be improved on the stabilization of the chargeability thereof, the environmental safety, the durability thereof and so forth by adding silica fine particles, which have been treated with an aminosilane coupling agent independently or in combination with a silane coupling agent, for making the silica fine particles hydrophobic, to the toner. However, from the results of the studies made by the present inventors, it was proved that, because there are a number of hydrophilic Si--OH functional groups present on the surfaces of silica particles not yet treated, the coupling agent reacts with every --OH group, so that all the --OH groups may not be blocked when treating simply with a coupling agent. Further, since the --OH groups remain considerably on the surfaces of silica particles, these attempts have the defects that the influence of humidity may not completely be prevented and no countermeasure may be taken to environmental changes. Namely. in the case of using repeatedly a number of times or in the conditions of a high humidity, fog is produced due to the lowered chargeability and images are stained due to the toner flying.
(3) In Japanese Patent O.P.I. Publication No. 201063-1984, there is the description of an attempt to obtain a durable chargeability stable against the changes in environment by making use of a developer containing silicic acid fine powder treated with silicone oil having amine coupled to the side-chain thereof. However, from the results of the studies thereof made by the present inventors, it was proved that, because a viscous oil substance is applied to the surface of the silicic acid fine powder, the oil substance is apt to adhere to the surface of a photoreceptor, carrier particles and such a friction-electrifying member such as a developer carrying member and so forth, so that stains are apt to produced, cleanliness of the surface of photoreceptor is deteriorated, the frictional chargeability of toners is made instable and, further, the durability of toners is deteriorated.
Further, in the above-mentioned treated silica, an amine type compound is used. According to the studies made by the inventors, however, this usage is not sufficient from the viewpoint of positive chargeability. In the case of adding such a treated silica as mentioned above into toners, the frictional chargeability of the toner of its own is rather lowered, because the chargeability of these silica is lower than that of the toners. In addition to the above, when the toners are stirred together with carrier particles and the like in a developing chamber and are applied with a physical pressure so as to frictionally charge toners, the treated silica remaining on the surfaces of toners is apt to transfer to other friction-electrifying members to stain such members, therefore, the chargeability of the toners are deteriorated. In such a developer as mentioned above, the adhesion force generated by Coulomb energy between toner particles and carrier particles is lowered to make toners fly into a copying machine, so that images are also stained. Further, silica fine particles treated with a conventional amine compound are apt to disturb the chargeability of toners. Therefore, the frictional electrification efficiency is relatively low. With such a developer, a good performance may probably be achieved in the initial stage of starting the first operation though, in the initial stage of resuming a further coying operation some time after a series of continuous operations were done, a charging rate is slow in getting started. When resuming a copying operation, therefore, an image is fogged and toners are flown about. This phenomenon will be more serious in the conditions of a high humidity where the leakage of charge is apt to occur.
When the present inventors applied the above-described treated silica to an image forming process in which a cleaning is made with a cleaning blade, it was found that, no cleaning trouble occurred in the initial stage though, but the trouble was apt to occur as copying frequency is increased more.
Especially when such a treated silica was applied to an organic photoreceptor, it was found that the conventional treated silica was apt to adhere strongly to the surface of the photoreceptor, because the surface of the photoreceptor contained resin components and, therefore, that a cleaning trouble was raised by the adhesion. Particularly because of the fact that such a component as talc, which was contained in transfer paper, also adhered to the surface of the photoreceptor, the treated silica and the above-mentioned components together produced an adherent matter cohered to the surface of the photoreceptor. Therefore, an electrotype static latent image forming function is lost from the cohesive areas of the photoreceptor surface, so that an image may not satisfactorily be formed, so that a faded and unsharp image, that is so-called `a vignetted image`, comes out and, at the same time, a cleaning trouble is also raised, because such a cohered adhesive matter may not be cleaned up by a cleaning blade.
As described above, in the case of using a conventional surface-treated silica, the following defects are involved; (1) A satisfactorily stable positive-charge may not be applied in coping with the changes in environment; (2) The charging efficiency is low and the initial chargeability is slow in getting started; (3) A cleaning trouble is apt to occur; (4) The durability is deteriorated; and so forth.
In the conventional type developers, it has been hard to produce fine and flexible magnetic brushes. Therefore, the developability thereof is low and a development may not be achieved unless a latent image is rubbed rather forcibly with a magnetic brush. Accordingly, there have been the defects that a toner image which was developed once is rubbed with a magnetic brush and thereby the traces of the magnetic brush are produced on the image (that is a phenomenon of producing white streaks in the direction of rubbing the toner image with the magnetic brush); thereby a blurry image phenomenon is produced so as to trail black-lines from the trailing edge of the image; or thereby the gradation reproducibility and resolving power of an image are deteriorated. The above-described phenomena will be more serious if the fluidity of a developer is further lowered in the conditions of a high humidity.